Solid ammonium nitrate propellant containing alkali metal aminobenzoate combustion catalysts



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3,025,191 SOLE) AMIVIGNIIUM NITRATE PRUPELLANT QOJN- TATNING ALKAELE METAL AWNQEENZGATE QCrCtlt EBUSTH'ZJN CATALYSTS Edwin Ives, Whiting, End, asslgnor to Standard Oil Comparry, Chicago, Ill, a corporation of lindiana No Drawing. Filed Dec. 13, 1957, Ser. No. 7tl3,704 4 Claims. (Cl. 149-418) This invention relates to ammonium nitrate-type compositions and particularly a catalyst for promoting the combustion of ammonium nitrate containing compositions.

In gas generation and rocketry usages it is necessary that the gas affording composition develop gas at a uniform rate; in the art this is spoken of as burning at a uniform rate. In ammonium nitrate compositions which consist essentially of ammonium nitrate particles and an oxidizable organic material which permits the shaping of the composition into a definite configuration or grain is necessary to promote the combustion of the composition by the use of a combustion catalyst. The best known catalysts are the inorganic chromium compounds particularly, ammonium dichromate. In the military field the Prussian blues have attained eminence. These and other metallic catalysts have the drawback of forming oxides with very high melting points. It is thought that these solid oxide products in the combustion gases are the cause of nozzle erosion. Nozzle erosion results in erratic change in gas pressure within the gas generator or rocket motor.

An object of the invention is an ammonium nitrate composition. A particular object of the invention is an ammonium nitrate composition having low erosive characteristics. A further object is an ammonium nitrate composition whose burning rate is relatively insensitive to variations in burning chamber pressure. Other objects will become apparent in the course of the detailed description.

It has been found that an eminently suitable ammonium nitrate-type composition for gas generator and rocketry usage is obtained with ammonium nitrate as the predominant component between about 10 and 40 weight percent of oxidizable organic material and between about 0.5 and weight percent of alkali metal aminobenzoate as the combustion catalyst. This catalyst burns to form products which are either gas or vapors at the temperature existant in the burning chamber and these products have no appreciable nozzle erosive activity.

The combustion catalyst used in the composition of the instant invention is an alkali metal aminobenzoate or a mixture of these materials. A particularly suitable catalyst is the commercially available form known as sodium anthranilate, i.e. the sodium salt of orthoaminobenzoic acid, i.e. anthranilic acid. Sufficient catalyst must be introduced into the composition to promote the burning of the composition. The amount of catalyst used is also infiuenced by the rate of burning desired. The more catalyst present the faster the combustion of the composition. It is to be understood that the burning rate is also affected by the particular oxidizable organic material present. In general, the composition will contain between about 0.5 and 15 weight percent of the alkali metal aminobenzoate. (Hereinafter all percentages are to be understood as weight percent.) With the thermoplastic matrix formers or binders obtained from cellulose esters and oxygenated hydrocarbon plasticizers therefor between about 1 and 6% alkali metal aminobenzoate produces satisfactory burning rates for typical military gas generation and rocketry usages.

The improved composition of the invention contains ammonium nitrate as the major component. The ammonium nitrate may be either ordinary commercial ammonium nitrate such as is used for fertilizers. This commercial grade material contains a small amount of impurities and the particles are usually coated with moisture resisting material such as paraffin wax. Military grade ammonium nitrate which is almost chemically pure is particularly suitable. The ammonium nitrate is preferably in a finely divided particulate form which may be either produced by prilling or by grinding. The ammonium nitrate is the major component of the gas-generator composition and usually the composition will contain between about 65 and of ammonium nitrate.

In order to permit the shaping of the ammonium nitrate composition into definite configurations a matrix former or binder material is present. When ammonium nitrate decomposes free-oxygen is formed. Advantage of the existence of this free-oxygen is taken and oxidizable organic materials are used as the binders. These oxidizable organic materials may contain only carbon and hydrogen, for example, high molecular weight hydrocarbons such as asphalts or residuurns, and rubbers either natural or synthetic. Or, the oxidizable organic material may contain other elements in addition to carbon and hydrogen for example, Thiokol rubber and neoprene. The stoichiometry of the composition is improved, with respect to smoke production, by the use of oxygenated organic materials as the binders. The binder or matrix former may be a Single compound such as a rubber or asphalt or it may be a mixture of compounds. The mixtures are particularly suitable When special characteristics are to be imparted to the grain which cannot be obtained by the use of a single compound.

The multi-component binder or matrix former commonly consists of a polymeric base material and a plasticizer therefor. Particularly suitable polymeric base materials are cellulose esters of alkanoic acids containing from 2 to 4 carbon atoms such as cellulose acetate, cellulose acetate butyrate and cellulose propionate; the polyvinyl resins such as polyvinylchloride and polyvinyl acetate are also good bases; styrene acrylonitrile is an example of a copolymer which forms a good base material. In general the binder contains between about 15 and 45% of the particular polymeric base material.

The plasticizer component of the binder is broadly defined as an oxygenated hydrocarbon. The hydrocarbon base may be aliphatic or aromatic or may contain both forms. The oxygen may be present in the plasticizer in ether linkage and/or hydroxyl group and/or carboxyl groups; also the oxygen may be present in inorganic substituents particularly nitro groups. In general any plasticizer which is suitable for work with the defined poly mers may be used in the invention. Exemplary classes of plasticizers which are suitable are set out below.

It is to be understood that these classes are illustrative only and do not limit the types of oxygenated hydrocarbons which may be used to plasticize the polymer.

Di-lower alkyl-phthalates, e.g. dimethyl phthalate, dibutyl phthalate. dioctyl phthalate and dimethyl nitrophthalare.

Nitrobenzenes, e..g. nitrobenzene, dinitrobenzene, nitrotoluene, dinitrotoluene, nitroxylene, and nitrodiphenyl.

Nitrodiphenyl ethers, e.g. nitrodiphenyl ether and 2,4-dinitrodiphenyl ether.

Tri-lower alkyl-citrates, e.g. triethyl citrate, tributyl citrate and triamyl citrate.

Acyl tri-lower alkyl-citrates where the acyl group contains 2-4 carbon atoms, e.g. acetyl triethyl citrate and acetyl tributyl citrate.

Glycerol-lower alkanoates, e.g. monoacetin, triacetin, glycerol tripropionate and glycerol tributyrate.

Lower alkylene-glycol-lower alkanoates wherein the glycol portion has a molecular weight below about 200, e.g. ethylene glycol diacetate, triethylene glycol dihexoate,

triethylene glycol dioctoate, polyethylene glycol dioctoate, dipropylene glycol diacetate, nitromethyl propanediol diacetate, hydroxyethyl acetate and hydroxy propyl acetate (propylene glycol monoacetate).

Dinitrophenyl-lower al-kyl-lower alkanoates, e.g. dinitrophenyl ethylacetate, and dinitrophenyl amyloctoate.

Lower alkylene-glycols wherein the molecular weight is below about 200, e.g. diethylene glycol, polyethylene glycol (200), and tetrapropylene glycol.

Lower alkylene-glycol oxolates, e.g. diethylene glycol oxolate and polyethylene glycol (200) oxolate.

Lower alkylene-glycol maleates, e.g. ethylene glycol maleate and bis-(diethylene glycol monoethyl ether) maleate.

Lower alkylene-glycol diglycolates, e.g. ethylene glycol diglycolate and diethylene glycol diglycolate.

Miscellaneous diglycollates, e.g. dibutyl diglycollate, dimethylalkyl diglycollate and methylcarbitol diglycollate.

Lower alkyl-phthalyl-lower alkyl-glycollate, e.g. methyl phthalyl ethyl glycollate, ethyl phthalyl ethyl glycollate and butyl phthalyl butyl glycollate.

Di-lower alkyloxy-tetraglycol, e.g. dimethoxy tetra glycol and dibutoxy tetra glycol.

Nitrophenylether of lower alkylene glycols, e.g. dinitrophenyl ether of triethylene glycol and nitrophenyl ether of polypropylene glycol.

Nitrophenoxy alkanols wherein the alkanol portion is derived from a glycol having a molecular weight of not more than about 20 0. These may be pure compounds or admixed with major component bis(nitrophenoxy)- alkane.

A single plasticizer may be used or more usually two or more plasticizers may be used in conjunction. The particular requirements with respect to use will determine not only the polymer but also the particular plasticizer or combination of plasticizers which are used.

In addition to the basic components, i.e. ammonium nitrate binder and catalyst, the gas-generator propellant composition may contain other materials For example, materials may be present to improve low temperature ignitability, for instance oximes may be present or, asphalt may be present. Surfactants may be present in order to improve the coating of the ntirate with the binder and to improve the shape characteristics of the composition. Various burning rate promoters, which are not catalyst per se, may also be present.

The aromatic hydrocarbon amines are known to be gas evolution stabilization additives. Examples of these aromatic amines are toluene diamine, diphenyl amine, naphthalene diamine, and toluene triamine. In general the aromatic hydrocarbon amines are used in amounts between about 0.5 and 5 percent. While these aromatic hydrocarbon amines are elfective, for severe duties they are frequently not sulficiently effective alone. It has been found that extremely good stabilization is obtained when N-phenylmorpholine additive is used with an aromatic hydrocarbon amine. Because of the plasticizing power of the N-phenylmorpholine it is generally desirable to use the aromatic hydrocarbon amines as the primary stabilizing additive and the N-phenylmorpholine in an amount needed to obtain the specific stability. In general when aromatic hydrocarbon amines are present between about 0.1 and 1 percent of N-phenylmorpholine will be used.

Tests The compositions to be tested for burning rate, pressure exponent and other characteristics required by military specifications were prepared by first forming a homogeneous viscous liquid binder at a temperature of about 130 C. lacquer grade commercial cellulose acetate analyzing about 55% of acetic acid was the polymer base. Two plasticizers were used. The one plasticizer contained about2 parts of dinitrophenoxy ethanol and 1 part of bis(dinitrophenoxy)ethane. This plasticizer mixture was obtained naturally by the reaction of dinitrochlorobenzene and ethylene glycol in the presence of aqueous sodium hydroxide solution. The other plasticizer was acetyl triethyl citrate or triacetin. The binder was then cooled to about C. and the ammonium nitrate in fine particles, carbon black and toluene diamine was blended into the binder until a pasty mass was formed. At this time sodium anthranilate was introduced into the mixture and worked into the mass of material. The homogeneous pasty mass was extruded as rods suitable for use in burning rate tests. Other portions were molded into gasgenerator grains for use in large size gas-generation tests. The burning rate tests were conducted in a Crawford Bomb pressured at 1000 p.s.i. and 25 C. temperature.

Test 1.In this test the composition consisted of cellulose acetate 8%, the dinitrophenoxy mixture 7%, acetyl triethyl citrate 7%, carbon black 3%, toluene diamine 1%, ammonium nitrate 71%, and sodium anthranilate 3%. This composition had a burning rate of 0.08 inch per second and a pressure exponent of 0.45.

Test 2.-This composition contained cellulose acetate 12%, acetyl triethyl citrate 9%, dinitrophenoxy mixture 9%, carbon black 4%, toluene diamine 1%, ammonium nitrate 62% and sodium anthranilate 3%. This composition had a burning rate of 0.08 inch per second at 1000 p.s.i.

Test 3.This composition contained cellulose acetate 11%, triacetin 9.5%, dinitrophenoxy mixture 9.5%, carbon black 4%, toluene diamine 1%, ammonium nitrate 62% and sodium anthranilate 3%. The composition had a burning rate of 0.07 inch per second at 1000 p.s.i.

Thus having described the invention, what is claimed is:

1. A composition consisting essentially of ammonium nitrate as the predominant component, between about 1 and 6 weight percent of alkali-metal aminobenzoate, between about 20 and 35 weight percent of a binder consisting of a cellulose ester of an alkanoic acid containing 2 to 4 carbon atoms and an oxygenated hydrocarbon plasticizer therefor.

2. A composition consisting of (a) ammonium nitrate, (b) cellulose acetate, about 812%, (0) acetyl triethyl citrate, about 79%, (d) about 7-9% of an about 2:1 mixture of dinitrophenoxyethanol and bis(dinitrophenoxy)ethane, (e) carbon, about 34%, (f) toluene diamine, about 1% and (g) sodium anthranilate, about 3-4%.

3. A composition consisting essentially of between about 0.5 and 15 weight percent of alkali-metal aminobenzoate combustion catalyst, ammonium nitrate as the predominant component and between about 10 and 40 Weight percent of oxidizable organic binder material wherein said binder material consists of a polymeric base selected from the class consisting of cellulose esters of alkanoic acids containing from 2 to 4 carbon atoms, polyvinyl chloride, polyvinyl acetate, and styrene-acrylonitrile, and an oxygenated hydrocarbon plasticizer therefor.

4. The composition of claim 3 wherein said catalyst is sodium anthranilate.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A COMPOSITION CONSISTING ESSENTIALLY OF AMMONIUM NITRATE AS THE PREDOMINANT COMPONENT, BETWEEN ABOUT 1 AND 6 WEIGHT PERCENT OF ALKALI-METAL AMINOBENZOATE, BETWEEN ABOUT 20 AND 35 WEIGHT PERCENT OF A BINDER CONSISTING OF A CELLULOSE ESTER OF AN ALKANOIC ACID CONTAINING 2 TO 4 CARBON ATOMS AND AN OXYGENATED HYDROCARBON PLASTICIZER THEREFOR. 